In recent years, laser cutting apparatuses that transmit laser beams from laser oscillators to processing heads, which process workpieces composed of metal or an alloy, via optical fibers so as to cut the workpieces by using the laser beams have been developed. A laser that uses such an optical fiber transmits a solid-state laser beam (such as a fiber laser beam, a disk laser beam, or a semiconductor laser beam) via the optical fiber.
In the related art, because the converging ability of a YAG laser beam or a semiconductor laser beam transmittable through an optical fiber is low, a CO2 laser has been used for processing a workpiece. However, fiber lasers require less electrical energy for generating a laser beam than CO2 lasers and readily achieve the same or higher beam quality (i.e., the converging ability and linearity of the laser beam), as well as high output. Therefore, fiber lasers are becoming more and more popular.
In order to obtain a laser beam with a small converging spot and a large focal depth, Patent Literature 1 discusses a converging optical system that converges a laser beam generated by a laser light source with a desired focal depth. This converging optical system is configured to generate spherical aberration.
A laser beam tends to have higher quality (i.e., higher linearity) in the central area thereof than in a peripheral area thereof, and the quality decreases with increasing distance from the central area. Therefore, when a workpiece is being cut, the low-quality laser beam in the peripheral area is in contact with the cut edges of the workpiece, thus affecting the cutting quality (such as the cutting precision and the roughness).
In particular, when performing a process for cutting metal, including an alloy, by using a YAG laser beam (in the 1 μm band), it is sometimes not possible to cut a thick metal plate (with a thickness of several millimeters, for example, 8 mm or more). Even if cutting is possible, the quality is not as high as that in a cutting process using a CO2 laser beam (in the 10 μm band) in the related art.
Patent Literature 2 discusses a laser processing apparatus that uses an inversion optical system equipped with two pairs of cylindrical lenses so as to invert an inner area and an outer area of the energy density distribution, which has a Gaussian distribution in the cross section of the laser beam, relative to a plane including an optical axis of the laser beam. The pair of cylindrical lenses disposed at the upstream side in the traveling direction of the laser beam includes two identical cylindrical lenses that are arranged parallel to each other and joined together and separates the laser beam into two beams. The pair of cylindrical lenses disposed at the downstream side includes two cylindrical lenses that are arranged parallel to each other with a distance therebetween and collimates the two laser beams separated by the upstream pair of cylindrical lenses before making the laser beams enter a converging lens.